Could Leaky Fuel Line Cause a PO420?

[brendanr279]

Hey, guys.  2002 Forester S 2.5L, 150000 miles. 

 

Brief history: For about a year, on below-freezing-days I get a strong gas smell coming through the vents when I'm first running the car.  My PO420 code has been on for quite some time now as well, probably about 9 months (I've reset it twice. Comes back on).  I don't notice much issue with my fuel efficiency (about 25 mpg highway, 20 city), my power seems okay, kind of "clunky"/"hesitant" sometimes, but I chalk that up to an old automatic transmission. 

 

I don't notice any obvious fuel leaks, the smell seems concentrated towards the air intake though when I sniff around closely under the hood.  I've covered my tailpipe and I feel pressure and also notice a pretty small exhaust leak at the downward end of the catalytic converter, no other exhaust leak, for what that's worth.

 

So my questions are: 

 

Could a small fuel leak be causing my PO420? 

 

Where might this fuel smell be coming from if it's in the area of the air intake? 

 

And I guess third, does measuring the before and after temperature of the catalytic converter give a good enough diagnostic to tell me that I just need a new catalytic converter?

 

Thanks for the help!

[montana tom]

Check how tight the lines at your fuel filter and fuel rail are. Common issue in cold weather. They leak when cold and stop leaking quickly as it warms up.  For code 420,  There is a fix out there that works at least 50% of the time. Buy 2 , 18 mm spark plug anti fouler s, drill one out with a 1/2" drill . Remove the rear o/2 censer. Screw it into the drilled out anti foul... gently tighten. Screw that into the undrilled anti foul & again gently tighten (this is aluminum not stainless steel) Screw that whole assembly into your cat.   Clear code and see if it comes back....    This keeps the rear o/2 warm and so the temp reading to the ecm does not spike. Its a cheap fix when it works. 

[Rooster2]

Yes,  PO420 code could come a fuel leak. I wouldn't be surprised if the leak is at the fuel filler pipe, where fuel is pumped into the tank. I had to replace both on my 98 and 99 Outbacks. That filler pipe starts leaking because of rust, when wet road crud gets trapped between it, and the plastic covering shield that surrounds it. Suggest removing the plastic shield to inspect the condition of the fill pipe for rust. Both of mine had severe rust spots that allowed gas vapor to be released. This was the cause of my PO420 code, as the vapor leak was not allowing proper vacuum within the fuel system. When replacing the fill pipes, I did not replace the plastic shielding. Replacement fill pipes can be ordered on line.

 

Suggest you drain and fill your auto tranny three times with new ATF. All the drain and refills are needed, as only half of the ATF can be drained at one time. The repeated fills does a good job of replacing old ATF.

[idosubaru]

Fix the fuel leak immediately. 

 

Find the exhaust leak and fix it if it's in front of any O2 sensors. 

 

In the end it doesn't matter and the P0420 is so common that proving it was from a past fuel leak on a 16 year old car in the rust belt with no known history is a tall order. 

[brendanr279]

Yes,  PO420 code could come a fuel leak. I wouldn't be surprised if the leak is at the fuel filler pipe, where fuel is pumped into the tank. I had to replace both on my 98 and 99 Outbacks. That filler pipe starts leaking because of rust, when wet road crud gets trapped between it, and the plastic covering shield that surrounds it. Suggest removing the plastic shield to inspect the condition of the fill pipe for rust. Both of mine had severe rust spots that allowed gas vapor to be released. This was the cause of my PO420 code, as the vapor leak was not allowing proper vacuum within the fuel system. When replacing the fill pipes, I did not replace the plastic shielding. Replacement fill pipes can be ordered on line.

 

Suggest you drain and fill your auto tranny three times with new ATF. All the drain and refills are needed, as only half of the ATF can be drained at one time. The repeated fills does a good job of replacing old ATF

 

You may be right, but I don't think it's coming from there.  The fuel smell seems to be concentrated behind the passenger side headlight, right by the air intake.  I'll take a closer look though.  Thanks.

[brendanr279]

Check how tight the lines at your fuel filter and fuel rail are. Common issue in cold weather. They leak when cold and stop leaking quickly as it warms up.  For code 420,  There is a fix out there that works at least 50% of the time. Buy 2 , 18 mm spark plug anti fouler s, drill one out with a 1/2" drill . Remove the rear o/2 censer. Screw it into the drilled out anti foul... gently tighten. Screw that into the undrilled anti foul & again gently tighten (this is aluminum not stainless steel) Screw that whole assembly into your cat.   Clear code and see if it comes back....    This keeps the rear o/2 warm and so the temp reading to the ecm does not spike. Its a cheap fix when it works. 

 

The fuel filter was my first suspicion, as I replace these myself.  But the connections are tight, and I don't smell much fuel around that area.  I tried chasing the fuel line but didn't inspect too close.  I'll take a closer look and tighten some clamps for good measure.

[Bushwick]

P0420 can be failing cats. Random misfires ignored for too long can can cause this. 

 

At idle and engine fully warmed up, try smelling the tail pipe. If it smells like a motorcycle or a 1960's muscle car, chances are the cat has come apart internally. A vacuum gauge can be a big help here as it'll affect engine vacuum if there is a clog. I've seen cats partially come apart before, and there might be enough flow to allow the engine to run, but it won't run right, especially when you floor it and get higher into the revs. It'll feel like the trans is fighting you (auto) through the power band, or engine might climb the revs poorly. IF the cat's ceramic melts and make it in the pipes, they typically end up in the mufflers, and will act as a further obstruction.